Substrate with photocatalytic coating

ABSTRACT

The invention concerns a method from cathode spray deposition of a coating with photocatalytic properties comprising titanium oxide at least partly crystallised in anastatic form on a transparent or semi-transparent support substrate, such as glass, vitroceramic, plastic. The substrate is sprayed under a pressure of at least 2 Pa. The invention also concerns the resulting coated substrate, wherein said substrate constitutes the top layer of a series of thin antiglare layers.

The invention relates to generally transparent or semi-transparentsubstrates, especially made of glass, plastic or glass-ceramic, andprovided with a coating having photocatalytic properties in order toendow them with an antistaining function or, more exactly, aself-cleaning function.

An important application of these substrates relates to glazing, whichmay be in very varied applications, such as utilitarian glazing, glazingused in domestic electrical appliances, windows for vehicles, andwindows for buildings.

It also applies to reflective glazing of the mirror type (mirrors fordwellings or rear-view mirrors for vehicles) and to opacified glazing ofthe apron wall or curtain walling type.

The invention also applies, similarly, to nontransparent substrates,such as ceramic substrates or any other substrate able in particular tobe used as an architectural material (metal, tiling, etc.). Itpreferably applies, whatever the nature of the substrate, tosubstantially plane or slightly curved substrates.

Photocatalytic coatings have already been studied, especially thosebased on titanium oxide crystallized in the anatase form. Their abilityto degrade stains of organic origin or microorganisms under the actionof UV radiation is greatly beneficial. They also often have ahydrophilic nature, which allows the removal of inorganic stains by thespraying of water or, in the case of exterior windows, by rain.

This type of coating with antistaining, bactericidal and algicidalproperties has already been described, especially in patent WO 97/10186,which describes several methods of obtaining them.

The aim of the invention is therefore to improve the techniques fordepositing this type of coating, especially for the purpose ofsimplifying them. In parallel, the aim of the invention is also toimprove the appearance of the coating, more particularly to improve theoptical properties of the substrate which is provided therewith.

The subject of the invention is firstly a process for depositing acoating having photocatalytic properties by sputtering, said coatingcomprising titanium oxide at least partly crystallized in the anataseform on a transparent or semitransparent carrier substrate. The featureof the invention consists in carrying out the sputtering on thesubstrate at a deposition pressure of at least 2 pascals. It ispreferably at most 6.67 Pa and especially at least 2.67 Pa (that is tosay at least 15 millitorr, especially between 20 and 50 millitorr).

In point of fact, as is known from the aforementioned patent WO97/10186, this type of coating can be deposited by sputtering. This is avacuum technique which in particular allows the thicknesses and thestoichiometry of the deposited layers to be very finely adjusted. It isgenerally enhanced by a magnetic field for greater efficiency. It may bereactive: in this case, an essentially metallic target is used, here atarget based on titanium (possibly alloyed with another metal or withsilicon), and the sputtering is carried out in an oxidizing atmosphere,generally an Ar/O₂ mixture. It may also be nonreactive: in this case atarget called a ceramic target is used, which is already in the oxidizedform of titanium (possibly alloyed).

However, the layers obtained by this type of technique are generallyamorphous, whereas the functionality of the coating according to theinvention is directly tied to the fact that it must be significantlycrystalline. This is the reason why, as is recommended in theaforementioned patent, it is necessary to crystallize (or increase thedegree of crystallization) of the coating by making it undergo a heattreatment, for example from about 30 minutes to several hours at atleast 400° C.

According to the invention, it has been shown that a pressure as high asthis favors particular crystallization of the layer and adensity/roughness level which have a significant impact on the level ofphotocatalytic properties of the coating. In some cases, the annealingmay become optional. To be specific, the deposition pressures generallyused for metal oxides are usually within the 2 to 8 millitorr (i.e. 0.27to 1.07 Pa) range: the invention therefore uses deposition pressureswhich are very unusual in this field.

It has also been shown within the context of the present invention thatthe post-deposition treatment step could possibly be eliminated, or atthe very least made optional (and/or limited in terms of time ortemperature), by sputtering the layer, not at ambient temperature, buton a hot substrate, especially heating to at least 100° C. This heatingduring deposition is alternative or cumulative with the abovementioneduse of high pressures.

This heating has at least five advantages:

-   -   a power saving during manufacture;    -   the possibility of using substrates which would be unable to        withstand heat treatments at temperatures of 400 or 500° C., at        least without degradation;    -   if the annealing requires the interposition between substrate        and photocatalytic coating of a barrier layer preventing the        diffusion of elements from the substrate (of the alkali metal        type when it is made of glass), the possibility of using a        thinner barrier layer, or even of completely dispensing with the        barrier layer, since the heat treatment according to the        invention is much less aggressive than an annealing operation;    -   a much shorter manufacturing cycle (since the heat treatment of        the substrate is substantially shortened and carried out at a        substantially lower temperature);    -   the need to store “semifinished” products to be annealed is        eliminated.

However, levels of photocatalytic activity in the coatings very similarto those of coatings which are deposited and then annealed are obtained.

However, this was not a foregone conclusion, insofar as one might haveexpected that a prolonged annealing operation would be indispensable formaking the crystalline seeds grow within the amorphous oxide matrix.This has not been the case: hot deposition favors direct deposition ofan at least partly crystallized layer.

Nor was it obvious that the coating thus deposited “hot” wouldpreferentially crystallize in the anatase form rather than in the rutileform (the anatase form is much more photocatalytic than the rutile orbroockite form of titanium oxide).

There are various alternative ways of implementing the invention,especially depending on the type of sputtering apparatus available.Thus, it is possible to heat the substrate prior to the actualdeposition, outside the vacuum chamber. It is also possible to heat thesubstrate during deposition, when the deposition chamber is fitted withad hoc heating means. The substrate may therefore be heated before thecoating is sputtered and/or while it is being sputtered. The heating mayalso be gradual during deposition, or may affect only part of thethickness of the deposited layer (for example the upper part).

Advantageously, while the layer is being sputtered, the substrate is ata temperature between 150 and 350° C., preferably at least 200° C. andespecially between 210 and 280° C. Surprisingly, it has therefore beenpossible to obtain sufficiently crystallized layers without having toheat the substrate up to the temperatures generally used to carry outannealing operations, namely at least 400° C. to 500° C.

In general, when the coating is essentially based on titanium oxide(TiO₂), and when it is deposited by sputtering (“hot” or at ambienttemperature), it has quite a high refractive index—greater than 2 orthan 2.1 or than 2.15 or 2.2. It is generally between 2.15 and 2.35 orbetween 2.35 and 2.50 (it may be slightly substoichiometric), especiallybetween 2.40 and 2.45. This is a feature quite specific to this type ofdeposition, since coatings of the same type deposited by othertechniques, for example by the sol-gel technique, tend to be much moreporous and have significantly lower refractive indices (below 2 and evenbelow 1.8 or 1.7). The invention makes it possible to obtain layers bysputtering which have a porosity and/or a roughness (especially an RMSroughness), of between 2.5 and 10 nm, enhancing its photocatalyticproperties. Consequently, they may have refractive indices of about 2.15or 2.35, less than those usually obtained by sputtering—indirect proofof their porosity. This is an asset from the optical standpoint, sincelayers with a lower refractive index have a less reflective appearancefor a given thickness.

It has been observed that the crystallographic structure of the coatingsis influenced by the fact that they are deposited cold and thenannealed, or deposited hot. Thus, quite unexpectedly, the coatingsdeposited “hot” and/or at high pressure, in accordance with theinvention, generally have a TiO₂ mean crystallite size generally lessthan or equal to 50 or 40 or 30 nm, especially between 15 and 30 nm orbetween 20 and 40 nm. Coatings deposited in a standard manner,especially “cold” and then annealed, tend to comprise crystallites oflarger size, namely at least 30 or 40 nm and generally between 40 and 50nm, when standard deposition pressures are used.

On the other hand, if, according to one variant of the invention, thecoating is deposited at ambient temperature but at high pressure, andthen an annealing operation is carried out, the size of the crystallitesis of smaller size (20-40 nm), comparable to that of the crystallites ofcoatings deposited hot, whether at high pressure or low pressure.

The photocatalytic activity of coatings deposited at ambient temperatureand at high pressure, and then annealed, is much better than that ofcoatings deposited at ambient temperature and at low pressure, and thenannealed: all other things being equal, it is clear that the depositionpressure has a pronounced influence on the performance of the coating,most particularly in the case of “cold” deposition.

Heating simultaneously with the growth of the layer results in theformation of a microstructure conducive to a roughness and/or porosityfavorable to the photocatalytic property. This is somewhat the same aswhen a high deposition pressure is used (with “cold” deposition followedby an annealing operation, for example).

Thanks to the process according to the invention (by hot and/orhigh-pressure deposition), it is possible to obtain coatings having anPMS (root mean square) roughness measured by atomic force microscopy,taking measurements over the same surface with a pitch of 2 micrometers:

-   -   of at least 2 nm, especially at least 2.5 nm and preferably        between 2.8 nm and 4.6 nm in the case of deposition at ambient        temperature and at high pressure within the meaning of the        invention (2 to 5 Pa), followed by annealing operations;    -   of at least 4 nm, especially at least 5 nm and preferably        between 5.5 and 6.0 nm in the case of hot deposition (at about        250° C.) without annealing, whether at high pressure or low        pressure.

By way of comparison, the roughness of the coatings deposited at ambienttemperature and at standard pressure (especially 2×10⁻³ millibars, i.e.0.2 Pa) and then annealed is only 2 nm at best: this proves that the useof high pressures makes it possible to achieve surprisingly highroughnesses for layers deposited by sputtering, this consequentlyimproving the photocatalytic properties of the coating.

Advantageously, the coating has a geometrical thickness of less than 150nm, especially between 80 and 120 nm or between 10 and 25 nm. It turnsout that the coating, even when very thin, can have sufficientlyphotocatalytic properties (at least for some applications) with, inaddition, the optical advantage of being hardly reflective.

As will have been seen above, the sputtering of the coating may bereactive or nonreactive. In either case, the target to be sputtered maybe doped, especially with at least one metal. This may be one or moremetals chosen from the following list: Nb, Ta, Fe, Bi, Co, Ni, Cu, Ru,Ce, Mo, Al.

The deposition process according to the invention may be preceded and/orfollowed by one or more steps of depositing one or more other thinlayers, especially with an optical, antistatic, anticolor,antireflective, hydrophilic or protective function, or to increase theroughness of the coating having photocatalytic properties. Thus, it hasbeen observed that there may be advantage in depositing one layer (atleast) so that it is particularly rough, for example by pyrolysis or bysol-gel, and then the photocatalytic coating; the coating then tends to“follow” the roughness of the underlayer and in fact to have, it too, asignificant roughness, whereas layers deposited by sputtering haveinstead a tendency to be not very rough. Thus, it is possible to formmultilayers with a sublayer (having an RMS roughness of, for example, atleast 5 or 10 nm) of the SiO₂, SiOC or SiON type, deposited by chemicalvapor deposition (CVD), and then the photocatalytic layer by sputtering.

The invention therefore comprises any combination of deposition of oneor more layers by sputtering (including at least the photocatalyticcoating) and deposition of the other layer(s) of the multilayer by atechnique involving thermal decomposition, especially pyrolysis (inliquid, vapor or pulverulant phase), or a sol-gel technique.

As was seen above, photocatalytic TiO₂-based coatings have a highrefractive index. This means that they are reflective and endow theircarrier substrate with a reflective appearance often regarded as notbeing very esthetically attractive. Apart from this shiny character, thecolor in reflection may also be undesirable. It is not simple to improvethis appearance in reflection, since the photocatalytic functionalityimposes constraints—the coating must in general be in contact with theexternal atmosphere in order to receive UV radiation and degrade theexternal stains. It therefore cannot be covered with a low-index layer(unless this is very thin and/or porous). It must also have a specificminimum thickness in order to be sufficiently effective.

Another part of the present invention has therefore consisted inimproving the appearance of the substrate in reflection, withoutdisturbing the photocatalytic activity of the coating, especially bylowering its light reflection as much as possible and/or by giving it acolor in reflection which is as neutral as possible.

The subject of the invention is therefore also the transparent orsemitransparent substrate defined above, provided over at least part ofat least one of its faces with a photocatalytic coating comprisingtitanium oxide at least partly crystallized as anatase, this coatinghaving a high refractive index, of at least 2 or 2.1 or 2.2. Accordingto the invention, this coating is regarded as forming part of amultilayer consisting of thin antireflection layers, the coating beingthe final layer (that is to say the layer furthest from the carriersubstrate). The antireflection multilayer is composed of an alternationof high-index and low-index layers and is therefore completed in thepresent case with the layer having a high photocatalytic index. Thisterm “antireflection” is used for convenience: in general, it isemployed when it is desired to obtain a light reflection less than thatwhich the substrate alone would have. Within the context of theinvention, it is more a question of limiting the increase in lightreflection (and/or modifying or attenuating its color in reflection)caused by the use of a coating containing titanium oxide.

Within the context of the invention, the term “layer” is understood tomean a single layer or a superposition of layers. If it is asuperposition of layers, its overall thickness is regarded as the sum ofthe thicknesses of each of the layers and its overall index is regardedas the average of all of the refractive indices of said layers. Thisalso applies to the photocatalytic coating. It may also be associatedwith another high-index layer.

Within the context of the invention and as recalled above, the term“antireflection” is understood to mean the function which makes itpossible to lower the light reflection value of the coated substrateand/or to attenuate its color in reflection, especially so as to make itas pale and as neutral as possible, i.e. as esthetically attractive aspossible (in this case one also speaks of an “anticolor” effect).

This is a quite free and unexpected adaptation of conventionalantireflection multilayers. This is because, in a known manner, thesemultilayers alternate high-index and low-index layers and are completedwith low-index layers (the index being as close as possible to therefractive index of air, equal to 1) and are generally layers based onSiO₂, MgF₂, etc. However, in the present case, the multilayer iscompleted with a high-index layer, something which is quite paradoxical.Nevertheless, by appropriately selecting the characteristics of thevarious layers, this particular antireflection multilayer is able tosignificantly attenuate the reflective nature intrinsic to high-indexTiO₂ and to give the substrate an acceptable color in reflection(neutral, in pale tints avoiding reds and other hot colors, deemed notvery esthetically attractive, in favor of gray, blue or especiallygreen).

Advantageously, the photocatalytic coating has a refractive indexgreater than or equal to 2.30, especially between 2.35 and 2.50, orbetween 2.40 and 2.45 (as seen above, it is also possible to deposit itso that it has an index of only 2.10 to 2.30). It is preferablydeposited by sputtering. Its optical thickness, in conjunction with thethicknesses of the other layers of the multilayer, is advantageouslyselected so as to reduce the light reflection of the substrate. It hasbeen shown that the optimum optical thickness is preferably in theregion of λ/2, where λ is about 580 nm. This corresponds to an opticalthickness of between 250 and 350 nm, especially between 270 and 310 nm,and to a geometrical thickness of between 80 and 120 nm, especiallybetween 90 and 110 nm. This geometrical thickness range has provedsufficient to obtain, in parallel, a photocatalytic activity regarded assufficient (the photocatalytic activity depends in fact on numerousparameters, including the thickness but also the surface roughness, thecrystalline morphology of the layer, its porosity, etc.). It is alsopossible to use substantially thinner layers, having in particular ageometrical thickness between 10 and 25 nm.

Depending on whether the coating is deposited by “hot” sputtering orsputtering at cold ambient temperature and annealing, it containscrystallites varying in size as was seen above (generally less than 30nm when sputtered “hot” and about 30 to 50 nm or more when sputtered atambient temperature and at standard pressure, as seen above).

The antireflection multilayer of the invention, in its simplestembodiment, comprises three layers, these being, in succession, ahigh-index layer, a low-index layer and then the high-indexphotocatalytic coating.

The high-index layer(s) of the multilayer, apart from the photocatalyticcoating, has (have) in general an index of at least 1.9, especiallybetween 1.9 and 2.3 or between 1.9 and 2.2. Said layer(s) may be made ofzinc oxide, tin oxide, zirconium oxide, aluminum nitride or siliconnitride. It (they) may also be made of a mixture of at least two ofthese compounds.

The optical thickness of these high-index layers is selected. Theiroptimum optical thickness is preferably in the region of λ/10, where λis about 580 nm. This corresponds to an optical thickness of between 48and 68 nm, especially between 53 and 63 nm, and to a geometricalthickness of between 20 and 40 nm, especially between 25 and 35 nm. Itis also possible to choose a smaller thickness, especially between 20and 48 nm.

The low-index layer(s) has (have) in general an index of between 1.4 and1.75, especially between 1.45 and 1.65. They may, for example, be basedon silicon oxide, aluminum oxide or a mixture of the two. The opticalthickness of these low-index layers is selected: their optimum opticalthickness is preferably in the region of λ/20, where λ is about 580 nm.This corresponds to an optical thickness of between 20 and 79 nm,especially between 19 and 39 nm, especially between 25 and 35 nm, and toa geometrical thickness of between 12 and 50 nm, especially between 15and 30 nm, for example between 20 and 28 nm.

According to another variant, in the abovementioned three-layermultilayer, it is possible to replace the high-index layer/low-indexlayer sequence with a layer having an “intermediate” refractive index,that is to say one preferably greater than 1.65 and less than 1.9. Thepreferred range of indices is between 1.75 and 1.85. It may be based onsilicon oxynitride and/or aluminum oxynitride. It may also be based on amixture of a low-index oxide such as SiO₂ and at least one oxide ofhigher index, such as SnO₂, ZnO, ZrO₂, TiO₂ (the relative proportionbetween the oxides allows the index to be adjusted).

It is also possible to use this intermediate layer to replace the firstsequence—high-index layer/low-index layer—with a multilayer containingnot three but five or seven layers for example.

The optical thickness of this intermediate-index layer is selected. Theoptimum optical thickness is in the region of λ/4, where λ is about 580nm. This corresponds to an optical thickness of between 120 and 150 nm,especially between 125 and 135 nm, and to a geometrical thickness ofbetween 65 and 80 nm, especially between 68 and 76 nm.

As mentioned above, these various optical thickness selections take intoaccount the overall appearance of the substrate in reflection: endeavorsare made not only to lower the light reflection value RL but also togive it a tint which today is deemed to be esthetically attractive (thatis to say rather in cold colors than toward yellow or red) and has thelowest possible intensity. It is therefore necessary to find the bestcompromise so that, overall, the appearance of the substrate inreflection is better. Depending on the applications, preference may begiven more to lowering the value of Rl [sic] or more to selecting aparticular calorimetric response in reflection (for example quantifiedby the a* and b* values of the L,a*,b* colorimetry system or by thevalue of the dominant wavelength associated with the color purity).

Advantageously, all of the layers of the antireflection multilayer maybe deposited by sputtering, one after the other, on the same productionline.

According to an optional variant of the invention, it is possible toinsert, between the substrate and the antireflection multilayer, abarrier layer blocking the species liable to diffuse out of thesubstrate. These are, in particular, alkali metals when the substrate ismade of glass. For example, the barrier layer is based on silicon oxide(or oxycarbide): SiO₂ may be deposited by sputtering and SiOC, in aknown manner, by chemical vapor deposition (CVD). It preferably has athickness of at least 50 nm, for example between 80 and 200 nm. Whenchosen in this type of material, having a relatively low index (around1.45 to 1.55), it is in fact, generally, pretty much “neutral” from theoptical standpoint. Silicon oxide may contain minority elements,especially chosen from Al, C, N.

The subject of the invention is also glazing, especially single-glazing(a rigid substrate), laminated glazing and multiple glazing of thedouble-glazing type, which comprises at least one substrate coated inthe manner described above.

Said glazing preferably has, thanks to the antireflection effect of theinvention, a light reflection R_(L) (on the multilayer side) whichremains at most 20%, especially at most 18%. Preferably, this lightreflection has a pleasant tint in the blues or greens, with negative a*and b* values in the (L,a*,b*) colorimetry system and especially lessthan 3 or 2.5 in absolute values. The tint is thus a color both pleasingto the eye and pale, of low intensity.

The glazing may also include one or more other functional coatings(deposited by sputtering or pyrolysis or sol-gel), either on the sameface of the substrate provided with the photocatalytic coating, or onthe opposite face of this substrate, or on a face of another substrateassociated with the first in a glazing unit (double glazing or laminatedglazing). It is also possible to have a double-glazing unit of theglass/gas-filled cavity/glass type with, on the exterior face(s) of theglass pane(s), the photocatalytic coating and, on the internal faces(turned toward the gas-filled cavity), a multilayer containing one ortwo silver layers. The same type of configuration applies to laminatedglazing.

The other functional coating(s) may in particular be an antistaining,solar-protection, low-emissivity, heating, hydrophobic, hydrophilic,antireflection or antistatic coating or another photocatalytic coating,etc. Mention may especially be made of solar-protection orlow-emissivity multilayers consisting of one or more layers of silver,or nickel-chromium, or titanium nitride or zirconium nitride. In thecase of layers based on a metal nitride, it is possible to use a CVDtechnique.

The invention will now be described in greater detail, with nonlimitingillustrative examples.

Example 1 and Comparative Example 1 relate to the hot deposition ofphotocatalytic TiO₂ layers by sputtering.

EXAMPLE 1

The following were deposited on a clear silica-soda-lime glass 4 mm inthickness: an 80-nm SiOC first layer by CVD and then a 90-nmphotocatalytic TiO₂ second layer (it is also possible to substitute theSiOC layer with an Al:SiO₂ layer obtained by reactive sputtering from anAl-doped Si target).

The TiO₂ layer was deposited by magnetic-field-enhanced sputtering. Thisis reactive sputtering, in the presence of oxygen, from a titaniumtarget. The glass was preheated to a temperature of about 220° C. to250° C. This temperature was kept constant to within 5° C. duringsputtering of the layer, using a heater placed opposite the target.

The TiO₂ layer obtained had a refractive index of 2.44. It crystallizedin the anatase form (it may also include amorphous regions), with anaverage crystallite size of less than 25 nm.

Its photocatalytic activity was quantified by means of a test usingpalmitic acid: this consists in depositing a given thickness of palmiticacid on a photocatalytic coating, in exposing the latter to UV radiationcentered on 365 nm with a surface power density of about 50 W/m²throughout the entire duration of the test, and then in measuring therate of disappearance of the palmitic acid according to the followingequation:${V\quad\left( {{nm}\text{/}h} \right)} = \frac{\left\lbrack {{palmitic}\quad{acid}\quad{thickness}\quad({nm})} \right\rbrack}{\left\lbrack {2\quad t_{{1/2}\quad{disappearance}}\quad(h)} \right\rbrack}$

With the layer according to the invention, a photocatalytic activity ofat least 10 nm/h, especially at least 20 nm/h, especially between 20 and100 nm/h, depending on the choice of the deposition parameters of thepressure and temperature type, was obtained using this calculation.

The glass thus coated with the two layers had, under illuminant D₆₅, alight reflection R_(L) of 23%, with a* and b* values in reflection inthe (L,a*,b*) colorimetry system of about 17 and 28, respectively.

The photocatalytic activity of the layer is therefore useful, but itsoptical appearance is still clearly reflective, with too intense acolor.

It should be noted that it is possible to increase the photocatalyticactivity of the layer by subjecting it, after deposition, to aconventional annealing operation (for one or several hours at at least400° C.).

COMPARATIVE EXAMPLE 1

Example 1 was repeated, but this time the TiO₂ layer was deposited on anunheated substrate and then treated for four hours at about 500 to 550°C. Furthermore, the SiO₂ sublayer was thickened to 100 nm. Themorphology of the layer was a little different, having a meancrystallite size somewhat greater than 30 nm.

Its photocatalytic activity was similar to that of the unannealed layerof Example 1, but it was less than it if a smaller thickness of SiO₂sublayer was chosen.

This therefore confirms that the “hot” deposition according to theinvention, making it possible to “save” on an often lengthy annealingoperation, is not obtained to the detriment of the performance of thelayer. This also confirms a subsidiary advantage of the invention: bydepositing hot and dispensing with an annealing operation, it ispossible to use, for identical photocatalytic performance, a thinnerbarrier sublayer (hence, again, resulting in a reduced productmanufacturing cost).

Example 2 and the following examples relate to the incorporation of aphotocatalytic layer of high-index TiO₂, said layer being especiallydeposited by sputtering, into antireflection multilayers in order toimprove the optical properties thereof.

EXAMPLE 2 Realized

The following multilayer stack was deposited on a silica-soda-lime floatglass 4 mm in thickness:

R_(L) (under illuminant D₆₅) = 17.3% a* (R_(L)) = −2 b* (R_(L)) = −2.8λ_(d) (dominant wavelength of the light 494 nm reflection) = ρe (purityof the color in 2.5%. reflection) =

The Si₃N₄ layer (1) was deposited by reactive sputtering in the presenceof nitrogen from an Al-doped Si target.

The SiO₂ layer (2) was deposited by reactive sputtering in the presenceof oxygen from an Al-doped Si target.

The photocatalytic TiO₂ layer (3) was deposited hot, as described inExample 1.

Optionally, it is possible to insert an additional layer, between theglass and the Si₃N₄ layer, [lacuna] a layer of SiO₂ of about 100 nmobtained, like the other SiO₂ layer (2) described above. It hasvirtually no influence on the optical properties of the substrate andmay serve as an alkali-metal barrier layer to the glass. This isoptional, all the more so since the layers of the antireflection coatingbeneath the photocatalytic layer, namely the layers (1) and (2),themselves constitute very satisfactory barrier layers, in addition totheir optical properties: these two layers already form a 100-nm barrierto the species liable to diffuse out of the glass.

The photocatalytic activity of the layer 3 was 80 nm/h.

Alternatively, a TiO₂ layer deposited cold and then annealed, asdescribed in Comparative Example 1, could have been used.

The results for such a multilayer, in reflection on the multilayer side,were the following:

glass/Si₃N₄ ⁽¹⁾/SiO₂ ⁽²⁾/TiO₂ ⁽³⁾ 30 nm 22 nm 104 nm (geometricalthicknesses).

This shows, compared with Example 1, a significant reduction in thevalue of R_(L) and in this case a paler color in the blue-greens isobtained. Overall, the appearance in reflection is thereforeesthetically and substantially improved.

EXAMPLE 3

This is very similar to Example 2, the only change being a slightreduction in the thickness of the TiO₂ layer.

Here, the following were deposited:

glass/Si₃N₄ ⁽¹⁾/SiO₂ ⁽²⁾/TiO₂ ⁽³⁾ 30 nm 22 nm 99 nm (geometricalthicknesses).

The results in light reflection were the following (with the sameconventions as for Example 2):

R_(L) = 17.9% a* = −0.8 b* = −0.7 λ_(d) = 494 nm ρe = 0.8%.

In this case, therefore, there is a slightly different compromise, witha slightly greater R_(L) value but a* and b* values lower in terms ofabsolute values.

EXAMPLE 4 Modeling

This is very similar to Example 2, the only change being the thicknessof the Si₃N₄ first layer:

glass/Si₃N₄ ⁽¹⁾/SiO₂ ⁽²⁾/TiO₂ ⁽³⁾ 25 nm 22 nm 104 nm (geometricalthicknesses).

The results in light reflection is the following (again with the sameconventions):

In this case, the value of R_(L) is greatly lowered, but the color inreflection changes tint.

EXAMPLE 5 Modeling/Comparative

Here, compared with Example 2, all the thicknesses are changed.

We have:

glass/Si₃N₄ ⁽¹⁾/SiO₂ ⁽²⁾/TiO₂ ⁽³⁾ 28 nm 30 nm 75 nm (geometricalthicknesses).

The results in light reflection are the following

R_(L) = 25.8% a* = −0.3 b* = −0.7 λ_(d) = 492 nm ρe = 0.5%.

Although the substrate has a satisfactory color in reflection, it doeshave, however, an R_(L) value well above 20%, which is too high: thechosen thicknesses are not optimal.

EXAMPLE 6 Modeling/Comparative

This example departs even further from the layer thicknesses recommendedby the invention, with the following multilayer:

glass/Si₃N₄ ⁽¹⁾/SiO₂ ⁽²⁾/TiO₂ ⁽³⁾ 20 nm 20 nm 60 nm (geometricalthicknesses).

The results in light reflection are the following:

R_(L) = 30% a* = 2.3 b* = 7.2 λ_(d) = 587 nm ρe = 14%.

The multilayer has both a very high R_(L) value and a not very desirableand more intense color in reflection. Its appearance in reflection istherefore unsatisfactory.

EXAMPLE 7 Realized

The stack this time was as follows:

glass/SnO₂ ⁽¹⁾/SiO₂ ⁽²⁾/TiO₂ ⁽³⁾ 30 nm 27 nm 105 nm (geometricalthicknesses).

Si₃N₄ has therefore been replaced with SnO₂, deposited by reactivesputtering in the presence of oxygen from a tin target.

The results in light reflection were the following:

R_(L) = 17.4% a* = −2.8 b* = −2.7 λ_(d) = 496 nm ρe = 2.8%.

The appearance in reflection is similar to that obtained in Example 2.

EXAMPLE 8 Modeled

Here, the first two layers are replaced with a single layer of silicogoxynitride SiON with an index of 1.84.

The multilayer is therefore the following:

glass/SiON/TiO₂ 72 nm 101 nm (geometrical thicknesses).

The results in light reflection are the following:

R_(L) = 17.4% a* = 0 b* = −1.08 λ_(d) = 480 nm ρe = 1%.

The appearance in reflection is therefore satisfactory.

EXAMPLE 9 Modeled

This repeats Example 8, but with a 1.86 index for the SiON layer.

The appearance in reflection is slightly modified therefrom:

R_(L) = 17.8% a* = −1.1 b* = −1.5 λ_(d) = 494 nm ρe = 1.3%.

EXAMPLE 10 Realized

The multilayer was the following:

glass/Si₃N₄ ⁽¹⁾/SiO₂ ⁽²⁾/TiO₂ ⁽³⁾/TiO₂ ⁽³⁾ 24 nm 17.5 nm 24 nm 92.5 nm

The final high-index “layer” was therefore the superposition of an Si₃N₄layer and a TiO₂ layer. The light reflection R_(L) on the multilayerside was between 16.5 and 17.5%, and the photocatalytic activity was inthe region of 80 nm/h.

EXAMPLE 11 Realized

The type of multilayer in Example 3 was repeated, but with differentthicknesses. The multilayer was:

glass/Si₃N₄ ⁽¹⁾/SiO₂ ⁽²⁾/TiO₂ ⁽³⁾ 14.5 nm 43 nm 14.5 nm

The light reflection on the multilayer side was between 13 and 16%. Ifeach of the layers of the stack were varied by 3%, the opticalvariations in the substrate thus coated were the following:

ΔR_(L): 0.8% Δa* (R_(L)): 0.3  Δb* (R_(L)): 1.3.

This example shows a photocatalytic activity of about 15 to 20 nm/h.

This example is useful on several counts: it is very insensitive tovariations in thickness and will therefore be easy to produce on anindustrial scale. It remains sufficiently photocatalytic, even thoughthe titanium oxide layer is very thin. It is satisfactory from thecalorimetric standpoint.

In conclusion, the invention has developed a novel way ofvacuum-depositing layers containing photocatalytic TiO₂. It has alsodeveloped a novel type of antireflection/anticolor multilayer which iscompleted with a high-index layer, said multilayer being simple toproduce on an industrial scale and significantly attenuating thereflective aspect of TiO₂ without degrading the photocatalyticproperties thereof. It makes it possible to obtain glazing in the bluesor in the pale greens in reflection, while maintaining consistentphotocatalytic layer thicknesses of the order of one hundred nanometers.It is also possible to choose a substantially thinner, 12-30 nm,photocatalytic layer.

The invention in its two aspects (product and process) may apply in thesame way to photocatalytic coatings which do not contain TiO₂.

The invention therefore proposes that these coatings be deposited “hot”and, alternatively, deposited at ambient temperature, followed byappropriate heat treatments, preferably with the deposition pressurebeing particularly controlled, in order to obtain vacuum-depositedlayers having very unusual characteristics, resulting in remarkableantistaining properties.

1. A process for depositing a coating having photocatalytic propertiesby sputtering, said coating comprising titanium oxide at least partlycrystallized in the anatase form on a transparent or semitransparentcarrier substrate of glass, glass-ceramic or plastic, wherein thesputtering is carried out at a deposition pressure P of at least 2 Paand at ambient temperature, the deposition of the coating beingoptionally followed by a heat treatment.
 2. The process as claimed inclaim 1, wherein the deposition pressure P is from 2.67 to 6.67 Pa. 3.The process as claimed in claim 1, wherein the process comprises saidheat treatment, and wherein the heat treatment is carried out at atemperature of at least 400° C.
 4. The process as claimed in claim 1,wherein the coating has a refractive index greater than
 2. 5. Theprocess as claimed in claim 1, wherein the coating comprises titaniumoxide crystallites having a size of less than or equal to 50 nm.
 6. Theprocess as claimed in claim 1, wherein the coating has a RMS roughnessof at least 2 nm.
 7. The process as claimed in claim 1, wherein thecoating has a geometrical thickness of less than 150 nm.
 8. The processas claimed in claim 1, wherein the sputtering is carried out in areactive manner from an essentially metallic target, or in a nonreactivemanner from a ceramic target.
 9. The process as claimed in claim 8,wherein the target to be sputtered is doped with a metal selected fromthe group consisting of Nb, Ta, Fe, Bi, Co, Ni, Cu, Ru, Ce, Mo, and Al.10. The process as claimed in claim 1, wherein the process is precededand/or followed by a step of depositing at least one layer by asputtering technique or by a technique comprising thermal decompositionby pyrolysis or by sol-gel, to increase the roughness of the coating.11. The process as claimed in claim 10, wherein the process is precededby the deposition of at least one layer by pyrolysis or by chemicalvapor deposition (CVD), said layer having a RMS roughness of at least 5nm.